{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"14d49bdf58cf221f616aae4985093a4bcbde329b4b4d43f1c6012cf748b9c503","created":"2026-10-03T07:18:05Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"c4f64b0dbcbe6455bcdb99bd7a5531bf7997f6dbb8bd7323821e48e32cb5df93","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"yes-no","assisted_by":[],"external_id":"hep.flavor-anomalies.cabibbo-anomaly","kind":"well-posed","literature_status":"open","n":"1","parents":[],"plain":"The probabilities for an up quark to turn into each kind of down-type quark should add to exactly one. Current data fall short by about a tenth of a percent, roughly 3 standard deviations.","posed_since":"2018","precise":"Test $\\left|V_{ud}\\right|^2 + \\left|V_{us}\\right|^2 + \\left|V_{ub}\\right|^2 = 1$, with $V_{ud}$ from superallowed $0+$ to $0+$ nuclear beta decays and neutron decay, and $V_{us}$ from K_l3, K_mu2/pi_mu2 and $\\tau$ decays. The current deficit is of order 1e-3 at about $3\\sigma$, together with an internal K_l3 versus K_mu2 tension; an answer is confirmation above $5\\sigma$ or resolution to below $2\\sigma$.","problem_ref":null,"references":"","settled_by":"Improved electroweak radiative corrections and nuclear-structure corrections for superallowed decays, new lattice QCD form factors for K_l3, and new K_l3/K_mu2 ratio measurements (NA62).","status_note":"A 2026 lattice-QCD analysis (arXiv:2604.04715) and related work still find a first-row deficit near $3\\sigma$.","title":"Is first-row CKM unitarity violated (Cabibbo angle 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